naturally turned to Mars as the most accessible, "Earth-like" extraterrestrial 

 object upon which to test this theory of chemical evolution. Before the Viking 

 mission it was assumed that organic materials, produced either photochemically 

 from the Martian atmospheric components or derived from meteoritic infall 

 from the neighboring asteroid belt, would be present on the Martian surface. 

 Indeed, one of the key objectives at that time was to ascertain the level of com- 

 plexity of these putative organic compounds. Did they include organics related 

 to terrestrial biological matter? Had chemical evolution on Mars advanced to the 

 point of producing replicating molecules? To biologists the search for life on 

 Mars was characterized by these and related questions, and not solely by a quest 

 for evidence of living or fossil "Martians." In retrospect, the Viking results 

 underscored how naive our assumptions were about the ease with which repli- 

 cating organic systems are formed and evolve on a planetary body. The results 

 emphasize how much additional knowledge we need about chemical and bio- 

 logical evolution. 



These uncertainties, of course, are applicable when contemplating the origins 

 of life on our own planet. The answers to many of the key questions in our 

 scenario of chemical evolution still elude us. How and where did the carbon- 

 containing precursors of terrestrial biology arise? What contributions to the 

 inventory of such precursors were made by comets, asteroids, or cosmic dust 

 during or after the accretion of the Earth? Indeed, what is the history of the bio- 

 genic elements themselves? Can we trace their origins and subsequent interac- 

 tions back through pre-solar system epochs to cosmic dust and interstellar 

 gases? During the past three decades, considerable strides have been made by 

 chemists and biochemists in demonstrating the relative ease with which the 

 biogenic elements can condense to form complex, biologically important organic 

 compounds. However, we are on tenuous ground when we try to pin down these 

 chemical processes in time and space. How much of the process took place on 

 this planet? What relationships exist between the organic compounds found in 

 the interstellar medium and those found in meteorites, comets, other solar 



system objects, and the Earth? 



The open issues regarding the accumulation of the organic material that set 

 the stage for the appearance of replicating molecules on Earth are further 

 obscured because our models for the early Earth preceding and during the period 

 when organic chemical evolution "went critical" (i.e., gave rise to reproducing 

 chemicals) are relatively imprecise. Not only is it important to understand the 

 physical environment within which replicating systems first appeared, but such 

 knowledge is equally necessary to gain insight into the earliest stages of biologi- 

 cal evolution— that is, how life, once initiated on Earth, was sustained and 

 became diversified. Uncertainties exist in specifying the history of the terrestrial 

 atmosphere. How, and over what time span, did the Earth's volatiles accumu- 

 late? Was the atmosphere strongly reducing or more oxidized at various stages in 



